Clinical, animal experimental and human genetic information currently support a relationship of serotonin ( 5- HT) to pulmonary hypertension (PH). Studies by us and others have indicated the importance of both the 5- HT transporter (5-HTT) and 5-HT receptors (5-HTRs) in the pulmonary vascular smooth muscle cell (SMC) proliferative response to 5-HT. We have identified collaborative actions of the 5-HTT and 5-HTRs through independent stimulations of signaling pathways in the proliferative response. MARK is activated through the 5-HTT and activation of the Rho pathway via 5-HT 1B participates in translocation of MARK from SMC cytoplasm to nucleus. More recently, we have also found that 5-HT transactivates the PDGF receptor (R) and interference of this activation by Gleevec, a tyrosine kinase inhibitor, blocks 5-HT-induced SMC proliferation. The Rho pathway plays a central role in 5-HT-induced SMC proliferation and has been reported to participate in hypoxia-induced PH;importantly, agents are currently available to modify this pathway. We hypothesize that activation of the Rho pathway and transactivation of PDGFR participate in SMC proliferative and migratory processes important in development of PH. We wish in this proposal to better study the 5-HT-related signaling pathways in vitro and in vivo that orchestrate SMC proliferation and migration. To do this we plan to: 1) Further define upstream and downstream effectors of 5-HT-induced Rho signaling in SMCs in culture that lead to SMC proliferation and migration;2) Determine the effect and mechanisms of action of statins and other geranylgeranyl and farnesyl transferase inhibitors on the 5-HT- induced SMC responses;3) Evaluate the role of transactivation of PDGFR by 5-HT in SMC proliferative and migratory responses;and 4) Correlate the signaling pathways occurring in vitro with those operative in vivo with the use of wild type and 5-HTT and 5-HTR transgenic mice exposed to hypoxia and infused with 5-HT by osmotic pumps. We believe the overall experiments will provide a better appreciation of 5-HT-related cell signaling pathways that may be operative in PH and may allow new approaches to therapy of this disease.